This invention relates in general to electrophotography and, more specifically, to a novel electrophotographic imaging member and process for using the imaging member.
In the art of electrophotography, an electrophotographic imaging member containing a photoconductive insulating layer is imaged by first uniformly electrostatically charging the imaging surface of the imaging member. The member is then exposed to a pattern of activating electromagnetic radiation such as light which selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer.
A photoconductive layer for use in xerography may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. One type of composite photoconductive layer used in xerography is illustrated in U.S. Pat. No. 4,265,990 which describes a photosensitive member having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are supported on a conductive layer with the photoconductive layer capable of photogenerating holes and injecting photogenerated holes sandwiched between the contiguous charge transport layer and the supporting conductive layer, the outer surface of the charge transport layer is normally charged with a uniform charge of a negative polarity and the supporting electrode is utilized as an anode. Obviously, the supporting electrode may also function as an anode when the charge transport layer is sandwiched between the electrode and a photoconductive layer which is capable of photogenerating electrons and injecting the photogenerated electrons into the charge transport layer. The charge transport layer in this embodiment, of course, must be capable of supporting the injection of photogenerated electrons from the photoconductive layer and transporting the electrons through the charge transport layer.
Various combinations of materials for charge generating layers and charge transport layers have been investigated. For example, the photosensitive member described in U.S. Pat. No. 4,265,990 utilizes a charge generating layer in contiguous contact with a charge transport layer comprising a polycarbonate resin and one or more of certain aromatic amine compounds. Various generating layers comprising photoconductive layers exhibiting the capability of photogeneration of holes and injection of the holes into a charge transport layer have also been investigated. Typical photoconductive materials utilized in the generating layer include amorphous selenium, trigonal selenium, and selenium alloys such as selenium-tellurium, selenium-tellurium-arsenic, selenium-arsenic, and mixtures thereof. The charge generation layer may comprise a homogeneous photoconductive material or particulate photoconductive material dispersed in a binder. Other examples of homogeneous and binder charge generation layer are disclosed in U.S. Pat. No. 4,265,990. Additional examples of binder materials such as poly(hydroxyether)resins are taught in U.S. Pat. No. 4,439,507. The disclosures of the aforesaid U.S. Pat. No. 4,265,990 and U.S. Pat. No. 4,439,507 are incorporated herein in their entirety.
Photosensitive members having at least two electrically operative layers as disclosed above in, for example, U.S. Pat. No. 4,265,990 provide excellent images when charged with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely developed electroscopic marking particles. However, when the charge transport layer comprises a film forming resin and one or more of certain diamine compounds, difficulties have been encountered with these photosensitive members when they are used under certain conditions in copiers, duplicators and printers. For example, image deletion bands are observed in the form of a band of deleted print in copy images when an automatic xerographic imaging system is allowed to remain inactive for extended periods of time such as over a long holiday weekend. The severity of the problem appears to be proportional to the number of copies made immediately preceeding shut down and also to the length of time the system is allowed to remain at rest. This image deletion band seems to correspond to the area on the photoreceptor directly below the corotron charging device when the system is in a shut down mode and is believed to be a surface phenomenon which can recover if given a sufficient amount of recovery time.
For enclosed, slower speed systems where the residence time of an incremental segment of the photoreceptor beneath a corotron is greater than for high speed machines, a reduction of contrast potential, increased cycle down and lower initial charges are observed with continued cycling under inadequate ventilation conditions. When cycling down occurs, the surface charge and charge acceptance decrease as the dark decay increases in the areas exposed and the contrast potential for good images degrades and causes faded images. Dark decay is defined as the loss of charge on a photoreceptor in the dark after uniform charging. This is an undesirable fatigue-like problem resulting in lower initial charges that cannot be maintained during image cycling and is unacceptable for automatic electrophotographic copiers, duplicators and printers which require precise, stable, and a predictable photoreceptor operating range. Contrast potential is defined as the difference in potential between the background or light struck areas of a photosensitive member and the unexposed areas of a photosensitive member after exposure to a pattern of activating electromagnetic radiation such as light. Variations in conrast potential can adversely affect copy quality, especially in modern copiers, duplicators and printers which by their very nature require photoreceptor properties to meet precise operating windows. A decline in contrast potential variations can cause copies to not exist at all or appear too light and fuzzy. Moreover, this degradation of the photoreceptor in enclosed, slower speed systems appears to be a bulk phenomenon which is considered to be of a permanent nature. Control of both contrast potential and dark decay of photosensitive members is important not only initially but through the entire cycling life of the photosensitive members.
Although the electrophotographic imaging members described above produce excellent images, usage under certain conditions can cause cycle down and image deletion bands to form. This is particularly evident in electrophotographic imaging members containing charge transport layers comprising aromatic diamine molecules dispersed in a polymer matrix. Thus, the characteristics of photosensitive members comprising a conductive layer and at least two electrically operative layers, one of which is a charge transport layer comprising a film forming resin and one or more aromatic amine compounds, exhibit deficiencies which are undesirable in modern copiers, duplicators, and printers. Accordingly, there is a need for compositions and processes which impart greater stability to electrophotographic imaging systems which undergo periodic cycling.